利用CryptoAPI加/解密文件： 允许用户输入口令，根据用户输入的口令，加/解密文件， 要求： 由用户选择要加密或解密的文件 • 用户要输入口令， 用户没有输入口令，则用随机数产生密钥句柄

实现这个功能，需要使用Windows的CryptoAPI，在C/C++语言中调用相关函数。

首先需要包含头文件：

#include <windows.h>
#include <wincrypt.h>

然后定义一些辅助函数：

// 生成随机数
void GenerateRandom(BYTE* buffer, DWORD length) {
    HCRYPTPROV hProv;
    if (CryptAcquireContext(&hProv, NULL, NULL, PROV_RSA_FULL, CRYPT_VERIFYCONTEXT)) {
        CryptGenRandom(hProv, length, buffer);
        CryptReleaseContext(hProv, 0);
    }
}

// 从用户输入的字符串生成密钥句柄
HCRYPTKEY GenerateKeyHandleFromPassword(const char* password) {
    HCRYPTHASH hHash;
    HCRYPTKEY hKey;

    // 创建一个哈希对象
    if (!CryptCreateHash(CALG_MD5, 0, 0, 0, &hHash)) {
        return NULL;
    }

    // 将密码字符串添加到哈希对象中
    if (!CryptHashData(hHash, (BYTE*)password, strlen(password), 0)) {
        CryptDestroyHash(hHash);
        return NULL;
    }

    // 从哈希对象中生成密钥
    if (!CryptDeriveKey(0, CALG_RC4, hHash, 0, &hKey)) {
        CryptDestroyHash(hHash);
        return NULL;
    }

    CryptDestroyHash(hHash);
    return hKey;
}

// 加密或解密文件
BOOL CryptFile(const char* inputFile, const char* outputFile, const char* password, BOOL encrypt) {
    DWORD dwBlockLen;
    DWORD dwBufferLen;
    DWORD dwCount;
    BYTE* pbBuffer = NULL;
    BYTE* pbKey = NULL;
    HCRYPTKEY hKey = NULL;
    HCRYPTPROV hProv = NULL;
    HCRYPTHASH hHash = NULL;
    BOOL bRet = FALSE;

    // 打开输入文件
    HANDLE hInputFile = CreateFile(inputFile, GENERIC_READ, FILE_SHARE_READ, NULL, OPEN_EXISTING, FILE_ATTRIBUTE_NORMAL, NULL);
    if (hInputFile == INVALID_HANDLE_VALUE) {
        goto cleanup;
    }

    // 打开输出文件
    HANDLE hOutputFile = CreateFile(outputFile, GENERIC_WRITE, 0, NULL, CREATE_ALWAYS, FILE_ATTRIBUTE_NORMAL, NULL);
    if (hOutputFile == INVALID_HANDLE_VALUE) {
        goto cleanup;
    }

    // 获取缓冲区长度
    if (!CryptAcquireContext(&hProv, NULL, MS_ENH_RSA_AES_PROV, PROV_RSA_AES, CRYPT_VERIFYCONTEXT)) {
        goto cleanup;
    }
    if (!CryptCreateHash(hProv, CALG_SHA1, 0, 0, &hHash)) {
        goto cleanup;
    }
    if (!CryptHashData(hHash, (BYTE*)"Data buffer", 12, 0)) {
        goto cleanup;
    }
    dwBlockLen = 0;
    dwBufferLen = sizeof(dwBlockLen);
    if (!CryptGetHashParam(hHash, HP_HASHSIZE, (BYTE*)&dwBlockLen, &dwBufferLen, 0)) {
        goto cleanup;
    }
    dwBlockLen = (dwBlockLen + 7) / 8;
    dwBufferLen = 1024 * dwBlockLen;
    pbBuffer = (BYTE*)malloc(dwBufferLen);
    pbKey = (BYTE*)malloc(dwBlockLen);
    if (pbBuffer == NULL || pbKey == NULL) {
        goto cleanup;
    }

    // 生成密钥
    if (password != NULL && strlen(password) > 0) {
        hKey = GenerateKeyHandleFromPassword(password);
    }
    else {
        GenerateRandom(pbKey, dwBlockLen);
        if (!CryptImportKey(hProv, pbKey, dwBlockLen, NULL, 0, &hKey)) {
            goto cleanup;
        }
    }

    // 加密或解密文件
    while (ReadFile(hInputFile, pbBuffer, dwBufferLen, &dwCount, NULL)) {
        if (dwCount == 0) {
            break;
        }

        if (encrypt) {
            if (!CryptEncrypt(hKey, 0, FALSE, 0, pbBuffer, &dwCount, dwBufferLen)) {
                goto cleanup;
            }
        }
        else {
            if (!CryptDecrypt(hKey, 0, FALSE, 0, pbBuffer, &dwCount)) {
                goto cleanup;
            }
        }

        if (!WriteFile(hOutputFile, pbBuffer, dwCount, &dwCount, NULL)) {
            goto cleanup;
        }
    }

    bRet = TRUE;

cleanup:
    if (hInputFile != INVALID_HANDLE_VALUE) {
        CloseHandle(hInputFile);
    }
    if (hOutputFile != INVALID_HANDLE_VALUE) {
        CloseHandle(hOutputFile);
    }
    if (pbBuffer != NULL) {
        free(pbBuffer);
    }
    if (pbKey != NULL) {
        free(pbKey);
    }
    if (hKey != NULL) {
        CryptDestroyKey(hKey);
    }
    if (hProv != NULL) {
        CryptReleaseContext(hProv, 0);
    }
    if (hHash != NULL) {
        CryptDestroyHash(hHash);
    }

    return bRet;
}

最后，在`main`函数中调用这些函数即可：

int main() {
    char inputFile[MAX_PATH];
    char outputFile[MAX_PATH];
    char password[256];

    // 输入文件名和口令
    printf("Enter input file: ");
    fgets(inputFile, MAX_PATH, stdin);
    inputFile[strlen(inputFile) - 1] = '\0';

    printf("Enter output file: ");
    fgets(outputFile, MAX_PATH, stdin);
    outputFile[strlen(outputFile) - 1] = '\0';

    printf("Enter password: ");
    fgets(password, 256, stdin);
    password[strlen(password) - 1] = '\0';

    // 加密或解密文件
    BOOL encrypt;
    printf("Do you want to encrypt (1) or decrypt (0) the file? ");
    scanf_s("%d", &encrypt);

    if (!CryptFile(inputFile, outputFile, strlen(password) > 0 ? password : NULL, encrypt)) {
        printf("Error!\n");
        return 1;
    }

    printf("Done.\n");
    return 0;
}

注意，这个示例只使用了RC4加密算法和MD5哈希算法，实际应用中需要根据需要选择更安全的算法。